Scale variability of atmospheric surface layer fluxes of energy and carbon over a tropical rain forest in southwest Amazonia; 1 diurnal conditions

The aim of this study is to investigate the low-frequency characteristics of diurnal turbulent scalar spectra and cospectra near the Amazonian rain forest during the wet and dry seasons. This is because the available turbulent data are often nonstationary and there is no clear spectral gap to separate data into "mean" and "turbulent" parts. Daubechies-8 orthogonal wavelet is used to scale project turbulent signals in order to provide scale variance and covariance estimations. Based on the characteristics of the scale dependence of the scalar fluxes, some classification criteria of this scale dependence are investigated. The total scalar covariance of each 4-hour data run is partitioned in categories of scale covariance contributions. This permits the study of some statistical characteristics of the scalar turbulent fields in each one of these classes and, thus, to give an insight and a possible explanation of the origin of the variability of the scalar fields close to the Amazonian forest. The results have shown that a two-category classification is the most appropriate to describe the kind of observed fluctuations: "turbulent" and "mesoscale" contributions. The largest contribution of the sensible heat, latent heat, and CO2 covariance contributions occurs in the "turbulent" length scales. Mesoscale eddy motions, however, can contribute up to 30% of the total covariances under weak wind conditions. Analysis of scale correlation coefficient [r(Tvq)] between virtual temperature (Tv) and humidity (q) signals shows that the scale patterns of Tv and q variability are not similar and r(Tvq) <1 for all analyzed scales. Scale humidity skewness calculations are negative during the dry season and positive during the wet season. This suggests that different boundary layer moisture regimes occur during the dry and wet seasons

Measurements of soil respiration and simple models dependent on moisture and temperature for an Amazonian southwest tropical forest

Soil respiration plays a significant role in the carbon cycle of Amazonian tropical forests, although in situ measurements have only been poorly reported and the dependence of soil moisture and soil temperature also weakly understood. This work investigates the temporal variability of soil respiration using field measurements, which also included soil moisture, soil temperature and litterfall, from April 2003 to January 2004, in a southwest Brazilian tropical rainforest near Ji-Paraná, Rondônia. The experimental design deployed five automatic (static, semi-opened) soil chambers connected to an infra-red CO2 gas analyzer. The mean half-hourly soil respiration showed a large scattering from 0.6 to 18.9 µmol CO2 m-2 s-1 and the average was 8.0±3.4 µmol CO2 m-2 s-1. Soil respiration varied seasonally, being lower in the dry season and higher in the wet season, which generally responded positively to the variation of soil moisture and temperature year round. The peak was reached in the dry-to-wet season transition (September), this coincided with increasing sunlight, evapotranspiration and ecosystem productivity. Litterfall processes contributed to meet very favorable conditions for biomass decomposition in early wet season, especially the fresh litter on the forest floor accumulated during the dry season. We attempted to fit three models with the data: the exponential Q10 model, the Reichstein model, and the log-soil moisture model. The models do not contradict the scattering of observations, but poorly explain the variance of the half-hourly data, which is improved when the lag-time days averaging is longer. The observations suggested an optimum range of soil moisture, between 0.11

Comparative measurements of carbon dioxide fluxes from two nearby towers in a central Amazonian rainforest: the Manaus LBA site

Forests around Manaus have staged the oldest and the longest forest-atmosphere CO2 exchange studies made anywhere in the Amazon. Since July 1999 the exchange of CO2, water, and energy, as well as weather variables, have been measured almost continuously over two forests, 11 km apart, in the Cuieiras reserve near Manaus, Brazil. This paper presents the sites and climatology of the region based upon the new data sets. The landscape consists of plateaus dissected by often waterlogged valleys, and the two sites differ in terms of the relative areas of those two landscape components represented in the tower footprints. The radiation and wind climate was similar to both towers. Generally, both the long-wave and short-wave radiation input was less in the wet than in the dry season. The energy balance closure was imperfect (on average 80%) in both towers, with little variation in energy partitioning between the wet and dry seasons; likely a result of anomalously high rainfall in the 1999 dry season. Fluxes of CO2 also showed little seasonal variation except for a slightly shorter daytime uptake duration and somewhat lower respiratory fluxes in the dry season. The net effect is one of lower daily net ecosystem exchange (NEE) in the dry season. The tower, which has less waterlogged valley areas in its footprint, measured a higher overall CO2 uptake rate. We found that on first sight, NEE is underestimated during calm nights, as was observed in many other tower sites before. However, a closer inspection of the diurnal variation of CO2 storage fluxes and NEE suggests that at least part of the nighttime deficits is recovered from either lateral influx of CO2 from valleys or outgassing of soil storage. Therefore there is a high uncertainty in the magnitude of nocturnal NEE, and consequently preliminary estimates of annual carbon uptake reflecting this range from 1 to 8 T ha-1 y-1, with an even higher upper range for the less waterlogged area. The high uptake rates are clearly unsustainable and call for further investigations into the integral carbon balance of Amazon landscapes

Soil Co2 efflux in central Amazonia: Environmental and methodological effects

Soil respiration plays a significant role in the carbon cycle of Amazonian rainforests. Measurements of soil respiration have only been carried out in few places in the Amazon. This study investigated the effects of the method of ring insertion in the soil as well as of rainfall and spatial distribution on CO2 emission in the central Amazon region. The ring insertion effect increased the soil emission about 13-20% for sandy and loamy soils during the firsts 4-7 hours, respectively. After rainfall events below 2 mm, the soil respiration did not change, but for rainfall greater than 3 mm, after 2 hours there was a decrease in soil temperature and respiration of about 10-34% for the loamy and sand soils, with emissions returning to normal after around 15-18 hours. The size of the measurement areas and the spatial distribution of soil respiration were better estimated using the Shuttle Radar Topographic Mission (SRTM) data. The Campina reserve is a mosaic of bare soil, stunted heath forest-SHF and tall heath forest-THF. The estimated total average CO2 emissions from the area was 3.08±0.8 µmol CO2 m-2 s-1. The Cuieiras reserve is another mosaic of plateau, slope, Campinarana and riparian forests and the total average emission from the area was 3.82±0.76 µmol CO2 m-2 s-1. We also found that the main control factor of the soil respiration was soil temperature, with 90% explained by regression analysis. Automated soil respiration datasets are a good tool to improve the technique and increase the reliability of measurements to allow a better understanding of all possible factors driven by soil respiration processes

Water balance, nutrient and carbon export from a heath forest catchment in central Amazonia, Brazil

Carbon storage values in the Amazon basin have been studied through different approaches in the last decades in order to clarify whether the rainforest ecosystem is likely to act as a sink or source for carbon in the near future. This water balance, dissolved organic carbon (DOC) and nutrient export study were carried out in a micro-scale heath forest (Campina) catchment in central Amazonia, Brazil. For a 1-year study period (18 March 2007 until 19 March 2008), rainfall amounted to 3054mm; of which, 1532mm was evaporated by the forest (4.1mmday-1). Rainfall interception loss amounted to 15.6% of gross rainfall. Surface runoff amounted to 485mm, whereas another 1071mm was discharged as regional groundwater outflow. Accumulated DOC exports in surface runoff amounted to 15.3gm-2year-1, whereas the total carbon exported was 55.9gm-2. This is much higher than that observed for a nearby tall rainforest catchment in central Amazonia (DOC export<20gm-2). As Campina heath forest areas cover a significant proportion of the Amazon Basin, these differences in ecosystem hydrological carbon exports should be taken into account in future studies assessing the carbon budget for the Amazon Basin. Macro-nutrient exports were low, but those of calcium and potassium were higher than those observed for tall rainforest in the Amazon, which may be caused by a lower retention capacity of the heath forest ecosystem